Among the components of the protein synthesis apparatus are a set of tRNA molecules and a set of aminoacyl-tRNA synthetases, or activating enzymes. It is of considerable interest to know how a cell regulates the synthesis of these macromolecules so that it always has the optimal amount of each for efficient protein synthesis. There are indications that, in the bacterium Escherichia coli, gene organization plays a significant role in regulating the amounts of various tRNA species that are made. Some tRNA genes, for instance, are present in multiple copies to increase the amount of tRNA made, and others are co-transcribed with ribosomal RNA genes. At present, we do not know how often such strategies are used, because the chromosomal locations of many of the tRNA and aminoacyl-tRNA synthetase genes remain unknown. I propose an investigation combining biochemical and genetic techniques for the analysis of gene organization affecting these macromolecules. New genes will be sought, and then selected genetic regions will be analyzed in detail; the purpose is to uncover possible operons including tRNA or aminoacyl-tRNA synthetase genes. A second goal is to elucidate structure-function relationships in tRNA and aminoacyl-tRNA synthetase molecules. The phenylalanyl-tRNA synthetase is an alpha 2 beta 2 tetramer containing two dissimilar kinds of polypeptide chains; the binding site for phenylalaine may be on the alpha subunit. I will attempt to determine the subunit carrying the tRNA binding site, by isolation and biochemical analysis of a mutant with altered tRNA specificity.